1. Field of the Invention
This invention relates to a single-phase disk-type brushless fan motor employing a single-phase disk-type brushless motor which can start itself.
2. Description of the Prior Art
In recent years, brushless motors are widely used owing to their high reliability in addition to their characteristics as dc motors that a torque is high relative to their size and the controllability is high.
Particularly, disk-type brushless motors (also called flat motors) having an axial gap structure wherein an axial air gap is involved are widely used in office automation appliances because they can readily be flattened.
However, brushless motors have a drawback that they are expensive because they require, for each of phases thereof, a circuit (a driving or an energization controlling circuit) for switching energization of armature coils in response to a signal from a position detecting element such as a Hall effect element which detects an N (north) or S (south) magnetic pole of a magnet rotor.
Accordingly, it is unwise to use such an expensive brushless motor as a fan motor which is used to blow a wind for cooling an object.
In order to resolve this drawback, a single-phase (single-phase energized) brushless motor which requires only one position detecting element and a single energization switching circuit, and hence can be produced at a low cost, is used as a fan motor.
Such a single-phase brushless motor has a so-called dead point at an energization switching point at which no torque is produced.
Therefore, a single-phase brushless motor is normally provided with a cogging generating magnetic member (an iron piece is mainly used therefor) for generating a torque (cogging torque also called a reluctance torque) in addition to an electromagnetic torque generated by an armature coil and a magnet rotor (field magnet) in order to eliminate such dead points and to allow self-starting of the motor.
In a coreless motor, for example, the following methods for generating a cogging torque are known. Referring first to FIG. 1, a 6-pole field magnet or magnet rotor 2 having an alternate arrangement of 6 N and S magnetic poles or pole zones is mounted on a rotor yoke 1 in an opposing relationship to a stator yoke 5 with an air gap 4 left therebetween and with a pair of coreless armature coils 3 disposed in the air gap 4. In the motor of FIG. 1, the stator yoke 5 has at a face thereof opposing the field magnet 2 two inclined surfaces which thus define the complementarily inclined air gap 4. This method, however, has drawbacks that the motor is relatively inefficient because the air gap is relatively great and that the construction of the motor is complicated and hence costly.
Referring now to FIG. 2, another method is illustrated. In the motor of FIG. 2, an iron bar 6 is mounted on a stator yoke 5 and extends through each of a pair of coreless armature coils 3 disposed in a uniform air gap 4 defined by the stator yoke 5 and a field magnet or magnet rotor 2 on a rotor yoke 1. According to this arrangement, a magnetic flux will appear as seen in FIG. 3 and hence the field magnet 2 will stop at a position in which the iron bars 6 are each opposed to the center of one of the N and S poles of the magnet rotor 2. Accordingly, if the armature coils 3 are located so as to generate a turning torque in such a stopping position of the magnet rotor 2, a coreless motor which can start itself will be obtained.
However, the method as shown in FIG. 2 has a drawback that if the thickness of the iron bars 6 is increased in order to increase the cogging torque to assure self-starting of the motor, the cogging torque around the dead points will decrease because the magnetic flux 7 will act as shown in FIG. 4 around the dead points and the cogging torque will be combined magnetically with magnetic flux 7.
In order to obtain an ideal torque--angular rotor displacement curve, it is necessary to obtain a composite torque curve 8 as shown in FIG. 5. In FIG. 5, an armature coil torque (electromagnetic torque) curve by an armature coil is indicated by a curve 9 while a cogging (reluctance) torque curve by a cogging generating magnetic member is indicated by a curve 10. As apparent from the armature coil torque curve 9 and the cogging torque curve 10, the cogging torque should be a half of the armature coil torque in magnitude. By this means, the torque curve 8 composite of the armature coil torque and the cogging torque will exhibit a substantially uniform rotational torque over the entire range of rotation. In order to obtain such an ideal composite torque curve 8, a cogging generating magnetic member (such as the iron bars 6) must be designed correctly in size and location.
On the contrary, if a single-phase brushless motor constructed in such a principle as illustrated in FIGS. 1 and 2 has no inclined air gap nor iron bar for generating a cogging torque, a torque curve 9 as shown in FIG. 5 is obtained by an armature coil. This torque curve 9 indicates that such a single-phase brushless motor has a so-called dead point at an energization switching point 30 at which no torque is generated. As described hereinabove, if a position detecting element is located opposite a dead point when the magnet roter 2 is stopped, energization of the motor will result in generation of no turning torque. Accordingly, the motor is not suitable for practical use.
FIGS. 6 to 10 illustrate a fan motor in to which a typical single-phase brushless motor, wherein an ideal turning torque can be obtained, is used. FIG. 6 is a plan view of a single-phase disk-type brushless fan motor, FIG. 7 a vertical sectional view of the motor, FIG. 8 a bottom plan view of a magnet rotor of the motor, FIG. 9 an enlarged vertical sectional view of a stator of the motor, and FIG. 10 a plan view of a stator armature of the motor.
The single-phase disk-type brushless fan motor generally denoted at 11 includes a motor case 12A located at the center of a fan motor casing 12 having a substantially square shape in plan. A plurality of stays not shown extend between and interconnect the motor case 12A and the fan motor casing 12, and a same number of windows 16 for passing a wind therethrough are defined by the stays between the motor case 12A and the fan motor casing 12. The motor case 12A has a bearing holder or hub 31 formed at the center thereof and extending upwardly therefrom. A bearing 21 is securely fitted in the bearing holder 31, and a rotary shaft 20 is supported for rotation by means of the bearing 21. A rotary fan 17 is securely mounted at the top of the rotary shaft 20 and has a plurality of fan blades 15 formed integrally around an outer periphery of a body 18 thereof for blowing a wind axially toward and through the windows 16.
A rotor yoke 19 is secured to the back side of the rotary fan body 18, and a magnet rotor 2 is secured to the back side of the rotor yoke 19. The magnet rotor 2 may have 2P (P is an integer equal to or greater than 2) alternate N and S magnetic poles or pole zones, and in the arrangement shown, the magnet rotor 2 is an annular 6-pole magnetic rotor with each magnetic pole magnetized over an angular width of 60 degrees, as illustrated in FIG. 8.
A stator armature 32 is located in an opposing relationship to the magnet rotor 2 with an axial air gap 13 defined therebetween.
Thus, the single-phase disk-type brushless motor 11 is comprised mainly of the rotatably supported rotor yoke 19, the magnet rotor 2, the stator armature 32, and a position detecting element 24 which will be hereinafter described. The stator armature 32 includes a stator yoke 23 and a pair of coreless armature coils 43-1, 43-2 mounted on an upper face of the stator yoke 23 opposing to the magnet rotor 2. The armature coils 43-1, 43-2 are located at symmetrical positions spaced by an angle of 180 degrees from each other relative to the center of the stator yoke 23 as seen in FIG. 10. A position detecting element 24 such as a Hall effect element and a cogging generating magnetic projection 25 are also located on the upper face of the stator yoke 23. Meanwhile, a printed circuit board 22 is located on a lower face of the stator yoke 23. A spacing 37 for accommodating therein electric parts which constitute an energization controlling circuit (not shown) is defined below the circuit board 22.
The armature coils 43-1, 43-2 are of the coreless type and each have a pair of radially extending, magnetically active conductor portions 43a, 43b which contribute to generation of a torque and include an angular width equal to the angular width of each magnetic pole of the magnet rotor 2, that is, an angular width of 60 degrees, in order to constitute the motor 11 as a single-phase brushless motor.
The cogging torque generating magnetic projection 25 formed on the upper face of the stator yoke 23 has an angular width substantially equal to the angular width of each magnetic pole of the magnet rotor 2 (60 degrees in mechanical angle) and is located such that its radial center line between the angular width is displaced rearwardly by an electrical angle of 90 degrees (45 degrees in magnetic angle and also in mechanical angle) from the magnetically active conductor portion 43b of one of the armature coils, for example, the armature coil 43-2, in a direction of rotation of the magnet rotor 2 (in a direction indicated by an arrow mark A).
Meanwhile, the position detecting element 24 is located at a substantially central position between the magnetically active conductor portion 43b of the armature coil 43-1 and the magnetically active conductor portion 43a of the armature coil 43-2 and has three terminals 26 which extend through a cutaway portion 28 formed in the the stator yoke 23 and are electrically connected to the printed circuit board 22 underlying the stator yoke 23.
According to the single-phase disk-type brushless fan motor 11 having such a construction as described above, the magnet rotor 2 can start itself and rotate in the direction of the arrow mark A without fail with the only one position detecting element 24.
However, if the single-phase disk-type brushless fan motor 11 does not include such a cogging torque generating magnetic projection 25, a torque curve obtained by the armature coils 43-1, 43-2 will be such as denoted at 9 in FIG. 5, and this torque curve 9 apparently indicates a dead point at an energization switching point 30 at which no torque is generated.
Accordingly, if the position detecting element 24 stops at a position in which it opposes to a dead point when the magnet rotor 2 stops, energization of the motor 11 will not result in generation of a starting torque. Thus, such a motor is not suitable for practical use.
Therefore, the cogging torque generating magnet projection 25 is provided at such a specific location as seen in FIG. 10 in order to obtain a cogging torque curve 10 as shown in FIG. 5. The cogging torque as indicated by the cogging torque curve 10 is generated by an attracting force between the projection 25 and the magnet rotor 2 and is preferably about one half of the armature coil torque curve 9 in magnitude.
A composite torque between the torque curves 9 and 10 will be such as shown by the composite torque curve 8, and thus the single-phase disk-type brushless fan motor 11 will have no dead point.
However, the single-phase disk-type brushless fan motor 11 has a drawback that the cost of parts and the assembling man-hours are high because the projection 25 must be mounted on the upper face of the stator yoke 23. Besides, the cutaway portion 28 must be formed in the stator yoke 23 and electric parts located on the printed circuit board 22 underlying the stator yoke 23 must be connected to the armature coils 43-1, 43-2 on the stator yoke 23 by means of wires extending through the cutaway portion 28 formed in the stator yoke 23. In this instance, insufficient insulation of the stator yoke 23 may cause short-circuiting, and hence the motor is not suitable for mass-production.